Recently, in building materials, automobile components, cellular phones and electric products, a lot of parts with a metallic appearance by plating an injection-molding article of resin, and highly decorative parts by coating are being used. Being accompanied by increasing concerns about environment problems, plating liquids in a chemical bath in plating a resin, solvents discharged in a painting process, and influences that carbon dioxide affects environment are seen as a problem. In particular, attempts to prevent leak of plating liquids are necessary, farther, movements to regulate plating liquid itself have been coming out.
In such situations, as a metallic molded part replacing plating, there is proposed a laminate that metal deposition is done on a polyester film, and laminated on other material (Japanese Unexamined Patent Publication No. 2000-43212). However, since this proposal uses an ordinary biaxially stretched polyester film, it is not possible to produce a molded part with a complicated shape like plating an injection-molding article. Disclosed is an excellently decorative part that a metallic printing layer is formed on a polyester film, which in turn is thermally transferred to a molded article (Japanese Unexamined Patent Publication No. 2005-119043). However, since this proposal discards a polyester film after transferring the metallic printing layer, it is not economical and has a problem that environmental loads become large. Further, a proposal of a metallic decorative sheet that a metal thin film layer is formed on a formable film, and processed after lamination (Japanese Unexamined Patent Publication Nos. 2004-1243 and 2005-262447). However, these proposals do not satisfy the requirement of appearance after forming when a polyester film is used, and when an acryl type film is used emphasizing appearance, adhesion to the metal layer is not sufficient and a primer layer needs to be formed, and no satisfactory level is achieved.
On the other hand, several proposals of polyester film usable in such metallic formable film are done. First, a polyester film consisting of polyethylene terephthalate as a main constitutional component and containing other composition is disclosed (Japanese Unexamined Patent Publication No. 2000-94575). However, this proposal is far from demand characteristics on formability and appearance after forming as well. Next, a polyester film with excellent formability having a specific melting point and tensile elongation at break is also proposed (Japanese Unexamined Patent Publication No. 2001-72841). However, since a film of this proposal has too high strain stress in processing, it is difficult to carry out thermoforming precisely. Further, a film that polyethylene terephthalate and polybutylene terephthalate are mixed by 1:1 to provide formability is disclosed (Japanese Unexamined Patent Publication No. 2002-321277). However, it is difficult to obtain a beautiful metallic film by this film. Further, a polyester film for molded part having a specific melting point and adopting a specific film making condition is also disclosed (Japanese Unexamined Patent Publication No. 2003-211606). However, in a film of this proposal, metallic brightness after forming is lowered as expected, and appearance point of view is not sufficient.
In order to satisfy formability, decorative effect and smoothness, in a laminated film of three layers of A layer, B layer and C layer, a polyester film having formability by the B layer being an intermediate layer is proposed (Japanese Unexamined Patent Publication No. 2006-51747). However, this polyester film is inferior in interlayer adhesion between the B layer being an intermediate layer and the A layer, C layer being a surface layer, so that delamination occurs easily after processing.
Movements that a decorative sheet is used for a molded article as an alternative for coating are active. By using a decorative sheet, a method carrying out vacuum forming, vacuum pressure forming, plug assist forming or the like is used. In this case, however, in severe forming processes such as heating, pushing a metal mold and vacuuming, there are problems that surface is scared and surface gloss is lowered. Therefore, there is a proposal that a thermoformable masking film is laminated (Japanese Translation of PCT Publication No. 2001-514984).
However, since a masking film of this proposal is a non-oriented urethane film directly casting on a decorative sheet, hardness of film is too low, peel property after forming and surface conditions of a decorative sheet after forming are insufficient.
As a metallic easily formable decorative sheet, to prevent scar in forming, a formable laminate that a masking layer is laminated is proposed (U.S. Pat. No. 6,565,955). As a masking layer, a high elongation film such as polyester, nylon and polyurethane is proposed. However, in this proposal, since stress in forming is not sufficiently low, in forming a formable laminate, forming compliance is insufficient, and peel property after forming is bad, thus, there has been a problem that fragment of a protection film remains on the surface of a metallic easily formable decorative sheet.
It could therefore be helpful to provide a biaxially oriented polyester film preferably used as a metallic molded part because of having heat resistance and dimensional stability capable of forming a metal thin film, thermofomability and an excellent metallic appearance after forming, and no delamination between layers in a film after thermoforming. It could also be helpful to provide a biaxially oriented polyester film preferably used as a surface protection film of decorative sheets such as paint film because appearance of a molded article can be maintained beautifully when used as a surface protection film in forming a formable decorative sheet.